docs: def pair(self, array, stats):
          """
          :return (array, array_stats) if stats, else (array, None)
          """
          if len(self.weights) > 1 and stats:
              statnames, statfuncs = zip(*stats)
              array_stats = compute_stats2(array, statfuncs, self.weights)
          else:
              array_stats = None
          return array, array_stats